The driest, dullest entry in almost any garden how-to book is the advice on soil. It's a shame: The tale of soil is full of weird characters and fascinating processes we understand barely, if at all. It's also unfortunate because soil is the single most important factor determining success in your garden. Right now, this autumn minute, is the time to improve your soil. Right now, conveniently enough, is when you have a free supply of the ideal soil conditioner—fallen leaves.
Most books include that same old dreary litany about finding out whether your soil is clay or sand, with the added burdensome assignment of getting a soil test. The soil test, something I've never done, seems darkly reminiscent of a biopsy; you can learn a fair bit without a test. Just look around the neighborhood and note what plants are doing well in local conditions.
It's worth knowing what Mother Earth has dealt you in the dirt department. But there isn't much you can do about the type of soil you have—clay or sand. You can sit around despairingly running sand through your fingers or squishing sticky clay into balls, and, of course, you can complain forever and say how much nicer it would be to have a garden in Shropshire, England; Champaign-Urbana, Ill.; or Tallahassee, Fla.
What any gardener possesses is basically flour ground from rocks with organic frosting on top. What you can change pretty easily is the health and texture of that frosting—the structure of your topsoil. Structure refers to how the soil sticks together or doesn't. How clumps of soil cohere determines the availability of pore spaces for air and water to enter, and how easily roots can penetrate.
A major source of misery for me when I worked in New York City parks was seeing my fellow gardeners using a leaf blower to blast dead leaves off the planted beds. The useful foliage was then bagged and trucked to a landfill. First, the poor plants were getting Hurricane Ike-style winds—but without moisture. Second, it's fine to simply leave leaves on perennials. By spring, they'll have broken down into humus. The ideal would have been to run a lawnmower over leaf piles and put the chopped-up leaf litter around the plants. Lacking a working lawnmower, what I did was collect leaves in hidden piles to break down on their own over the winter and use in the spring or the following fall.
The roots, even of big trees, take in most of their moisture and nutrients from an amazingly thin layer of soil—less than a foot down and often only the first few inches. We used to think of soil as that brown substance useful for holding plants upright, into which we could inject fertilizer. Aside from a beneficial earthworm or troublesome mole, most gardeners didn't care much about what was living underground.
In fact, the fertility of those top inches depends on an incredibly complex society of small creatures—mites, beetles, sow bugs, water bears, nematodes, millipedes, springtails, fungi, and bacteria—that we're only beginning to understand. As Leonardo da Vinci observed, "We know more about the movement of celestial bodies than about the soil underfoot." It's only in the past couple of decades that soil ecologists have systematically applied microbiological techniques to learning more.
Magnified, soil mites are lobsterlike critters (like crustaceans, mites are arthropods) with impressive armor and scary chewing parts. Their job, along with the above-named allies, is to break down organic matter (like leaves, stems, insect corpses, and dung) into a soluble form that can be taken up by plants.
How crucial are they? Let me quote from a book with an amazing title: Life: A Natural History of the First 4 Billion Years of Life on Earth by Richard Fortey. Fortey is a paleontologist, and in a scoop of soil he can find mites hardly changed since the creatures of the Devonian Age. He is also a gorgeous writer:
If the spring tails were to undergo a mysterious demise, together with the mites that live in soil and the minute fungi upon which they feed, quite soon there would be an ecological crisis of a magnitude we can scarcely imagine. Nutrients would become locked away, the soil would become progressively impoverished, larger plants would die, and soon animals would follow suit. Novels that seek to portray post-holocaust worlds always seem to assume that the soil will magically survive, and that a bean cast into seared soil will quietly proceed to a successful crop. But the soil is not a passive medium; it is alive. I doubt whether there would be many readers for a post-holocaust novel that was concerned with the hero's desperate search for mites. Bu alas for the world if the mites and their diminutive allies failed to prosper!
Since the earth emerged from the sea, there has been a mutual dependence between plants and animals; the tiny animals feed the plants. The big animals, including us, eat the plants. Plants' roots, it turns out, aren't passive participants. They exude sugars, acids, and other compounds to attract or repel bacteria and fungi and stir up microbial action.
Autumn is the best time to keep the helpful creatures warm and fed, because they're probably at their most populous. The ground will retain warmth late into autumn, and the creatures will continue to decompose the organic matter. Autumn soil prepared with a couple of inches of compost and a couple more of mulch will protect the organisms from temperature extremes and keep those organisms active long into winter.
The best down quilt to lay on your planting beds is compost. Next-best: leaf mold from last autumn's leaves and, after that, this year's shredded leaves. And if you want to just go out and buy something, get bags of shredded bark.
One caveat: Don't use oak leaves. Part of the mighty oak's mightiness is that the leaves are slow to break down—they'll make a layer like Naugahyde. Other big leaves, like maple and tulip poplar, mat together and in northern gardens can freeze into a giant pancake that keeps water and air from the soil below.
You want to build your soil rather than dose it with fertilizer. Scary though the thought is, you actually want to participate in the cycle of decay and rebirth. The nutrients you give the critters go into the roots and up to the leaves. They return to the earth this time of year, when the leaves drift back down. Since we're all quoting FDR these days: Don't leave those all-important soil creatures ill-housed, ill-clothed, or ill-fed.